Image processing method to reduce marking material coverage in printing processes

Abstract

A method for processing images for printing includes the steps of for each separation, making a pixel level determination of marking material coverage, based on the number of pixels within a given area that are turned on; if a determination is made that too much marking material will be placed within the given area, turning off a fraction of pixels in the area to reduce the amount of marking material that will be used for the given area; and to preventing artifacts from occurring in the pixel reduction step by utilizing a processing order through each given area which tends to randomize the turn off effect. Additionally, the results of the determination are compared for each separation, to determine that the turnoff result allows at least one corresponding pixel among each of the separations to be turned ON.

I claim: 
     
       1. A method for processing a color image for printing to reduce an amount of marking material used therefor, the color image comprised of multiple separations each defined by a bitmap of single bit pixels each having a pixel value and arranged in an array of scan lines, including the steps of: for each separation bitmap, measuring a marking material coverage value over a given area within the image;   summing the marking material coverage of each separation bitmap to determine measured marking material coverage;   comparing desired marking material coverage with measured total marking material coverage to derive a ON/OFF ratio, where the ON/OFF ratio represents a number of pixels to be turned OFF, irrespective of their pixel value, compared to the number of pixels in a separation;   setting a number of pixels to OFF in the separation bitmap irrespective of their state, in a larger number of pixels in a separation, in accordance with the turn OFF ratio; and   outputting the separation bitmap to a print controller.   
     
     
       2. The method as defined in claim 1, wherein the step of measuring marking material coverage includes the sub steps of: directing pixel values of a set of k pixels from a single scan line of pixels into a latch, where k is an integer number of pixels;   providing a pre-loaded look up table, having a set of k bit addresses, each address corresponding to data entries including a possible combination of marking material coverage values for a set of k pixels;   storing to a memory, the marking material coverage values from the look up table corresponding to each set of k pixel values for a set of e scan lines; and   summing for the image, the marking material coverage values for each separation over the area of a set of k pixel values in a set of e scan lines.   
     
     
       3. The method as defined in claim 1, wherein the step of setting a number of pixels to OFF in the bitmap, irrespective of their state, in a larger number of pixels in a separation in accordance with the turn OFF ratio includes the sub steps of: incrementing a counter value by a predetermined amount related to the OFF/ON ratio, whereby increasing the counter value increases the likelihood of the pixel being set OFF;   comparing the counter value with a reference to determine whether to set the pixel OFF;   upon determining that a pixel should be turned OFF, resetting the counter so that the next pixel has a decreased probability of being turned OFF;   performing a masking process between the pixel value and the ON/OFF determination to generate the separation bitmap.   
     
     
       4. The method as defined in claim 1, wherein the step of comparing desired marking material coverage with measured marking material coverage to derive a turn OFF ratio, where the turn OFF ratio represents a number of pixels to be turned OFF, in a larger number of pixels in a separation includes the additional step of: determining the location in the image of pixels which should be turned off in accordance with the turn off ratio.   
     
     
       5. The method as defined in claim 4, wherein the step of determining the location in the image of pixels which should be turned off in accordance with the turn off ratio includes the substep of: processing the pixels within the given area in a predetermined order.   
     
     
       6. The method as defined in claim 5, wherein the predetermined order corresponds to the path of a space filling function through each pixel in the given area. 
     
     
       7. The method as defined in claim 4, wherein the step of determining the location in the image of pixels which should be turned OFF in accordance with the turn off ratio includes the additional step of: comparing the state of corresponding pixels in each separation bitmap and upon detection of a concurrence of pixels turned OFF, turning at least one pixel from said corresponding pixels, ON.   
     
     
       8. A method for processing a color image for printing by an ink jet printer to reduce ink used thereby, the color image comprised of multiple separations each defined by a bitmap of single bit pixels each having a pixel value and arranged in an array of scan lines, including the steps of: for each separation bitmap, measuring an ink coverage value over a given area within the image and summing the ink coverage of each bitmap to determine total ink coverage;   comparing desired ink coverage with measured total ink coverage to derive an ON/OFF ratio, where the ON/OFF ratio represents a number of pixels to be turned OFF, irrespective of their pixel value, compared to the number of pixels in a separation;   setting a number of pixels to OFF in the separation bitmap irrespective of their state, in a larger number of pixels in a separation, in accordance with the turn OFF ratio; and   outputting the separation bitmap to a print controller.   
     
     
       9. The method as defined in claim 8, wherein the step of measuring ink coverage includes the sub steps of: directing pixel values of a set of k pixels from a single scan line of pixels into a latch, where k is an integer number of pixels;   providing a pre-loaded look up table, having a set of k bit addresses, each address corresponding to data entries including a possible combination of ink coverage values for a set of k pixels;   storing to a memory, the ink coverage values from the look up table corresponding to each set of k pixel values for a set of l scan lines; and   summing for the image, the ink coverage values for each separation over the area of a set of k pixel values in a set of l scan lines.   
     
     
       10. The method as defined in claim 8, wherein the step of setting a number of pixels to OFF in the bitmap, irrespective of their state, in a larger number of pixels in a separation in accordance with the turn OFF ratio includes the sub steps of: incrementing a counter value by a predetermined amount related to the OFF/ON ratio, whereby increasing the counter value increases the likelihood of the pixel being set OFF;   comparing the counter value with a reference to determine whether to set the pixel OFF;   upon determining that a pixel should be turned OFF, resetting the counter so that the next pixel has a decreased probability of being turned OFF;   performing a masking process between the pixel value and the ON/OFF determination to generate the separation bitmap.   
     
     
       11. The method as defined in claim 8, wherein the step of comparing desired ink coverage with measured ink coverage to derive a turn OFF ratio, where the turn OFF ratio represents a number of pixels to be turned OFF, in a larger number of pixels in a separation includes the additional step of: determining the location in the image of pixels which should be turned off in accordance with the turn off ratio.   
     
     
       12. The method as defined in claim 11, wherein the step of determining the location in the image of pixels which should be turned off in accordance with the turn off ratio includes the substep of: processing the pixels within the given area in a predetermined order.   
     
     
       13. The method as defined in claim 12, wherein the predetermined order corresponds to the path of a space filling function through each pixel in the given area. 
     
     
       14. The method as defined in claim 11, wherein the step of determining the location in the image of pixels which should be turned OFF in accordance with the turn off ratio includes the additional step of: comparing the state of corresponding pixels in each separation bitmap and upon detection of a concurrence of pixels turned OFF, turning at lease one pixel from said corresponding pixels, ON.   
     
     
       15. A method for processing a color image for printing, to reduce an amount of marking material used thereby, the color image comprised of multiple separations each defined by a bitmap of single bit pixels each having a pixel value and arranged in an array of scan lines, including the steps of: receiving pixel values of a set of k pixels from a single scan line of pixels, where k is an integer number of pixels;   providing a pre-loaded look up table, having a set of k bit addresses, each address corresponding to data entries including a possible combination of marking material coverage values for a set of k pixels;   storing to a memory, the marking material coverage value from the look up table corresponding to each set of k pixel values for a set of l scan lines;   summing the marking material coverage values for the set of l scan lines to derive a marking material coverage value for the area of K pixels by l scan lines in each separation;   summing the marking material coverage values for the separations to derive a marking material coverage value for the image over an area of K pixels by l scan lines;   determining the location in the image of pixels which should be turned OFF in accordance with a turn OFF ratio;   incrementing a counter value by a predetermined amount related to the turn OFF ratio, whereby increasing the counter value increases the likelihood of the pixel being set OFF;   comparing the counter value with a reference to determine whether to set the pixel OFF, said comparing generating an ON/OFF determination signal;   upon determining that a pixel should be turned OFF, resetting the counter so that the next pixel has a decreased probability of being turned OFF;   performing a masking process between the pixel value and the ON/OFF determination signal to generate a separation bitmap; and   outputting each separation bitmap to a printer.   
     
     
       16. The method as defined in claim 15, wherein the step of determining the location in the image of pixels which should be turned off in accordance with the turn off ratio includes the substep of: processing the pixels within the area in a predetermined order.   
     
     
       17. The method as defined in claim 16, wherein the predetermined order corresponds to the path of a space filling function through each pixel in the area. 
     
     
       18. The method as defined in claim 15, wherein the step of determining the location in the image of pixels which should be turned off in accordance with the turn off ratio includes the additional step of: comparing the state of corresponding pixels in each separation bitmap and upon detection of a concurrence of pixels turned OFF, turning at least one pixel from said corresponding pixels ON.   
     
     
       19. A device for processing a color image comprised of multiple separations each defined by a bitmap of single bit pixels for printing, to reduce an amount of marking material used in the color image, including: means for predicting the marking material coverage of an area in an output image as a function of the input image;   means for determining a ratio of pixels to be turned OFF, irrespective of their state, as a function of a desired marking material coverage for a page on which the color image is printed and the predicted marking material coverage;   processing order control means, for determining a sequence of pixels processed in the input image for reducing marking material coverage;   turn off determining means for determining whether a pixel should be turned OFF in accordance with the turn OFF ratio, where each pixel is processed in an order controlled by the process order control means;   a masking circuit, having as inputs the pixels in each separation of the color image and the turn off ratio determination from said turn off determining means, and having as an output a pixel value for a reduced marking material image; and   output means for directing the pixel value to a print controller.   
     
     
       20. The device as defined in claim 19, wherein the turn off determining means for determining whether a pixel should be turned OFF in accordance with the turn OFF ratio includes: a counter, maintaining a count value incremented by a predetermined amount related to the turn OFF ratio for each pixel processed, whereby increasing the counter value increases the likelihood of the pixel being set OFF;   means for comparing the count value with a reference to determine whether to set the pixel OFF; and   means for resetting the counter so that the next pixel has a decreased probability of being turned OFF.   
     
     
       21. The device as defined in claim 19, wherein the turn off determining means for determining whether a pixel should be turned OFF in accordance with the turn OFF ratio includes: means for comparing the state of corresponding pixels in each separation bitmap and upon detection of a concurrence of pixels turned OFF, turning at least one pixel from said corresponding pixels, ON.